The present invention concerns a method and apparatus for preventing vehicle handling instabilities, in which a vehicle yaw angular velocity required value (.mu..sub.soll) is formed from measured quantities (vehicle speed, steering wheel angle) in a computer unit supplied with at least one sensor signal from which the actual value of the vehicle yaw angular velocity (.mu..sub.ist) is formed. The difference between the yaw angular velocity required value (.mu..sub.soll) and the yaw angular velocity actual value (.mu..sub.ist) is formed in the computer unit by subtracting the yaw angular velocity actual value (.mu..sub.ist) from the yaw angular velocity required value (.mu..sub.soll), and at least one output signal formed in the computer unit from this difference and is output from the computer unit as representing the detected handling situation or the yaw behavior of the vehicle.
A method of preventing vehicle handling instabilities is described in DE 36 25 392 A1 in which, in order to detect the handling situation or the yaw behavior of the vehicle, the vehicle yaw angular velocity (.mu..sub.ist) is measured by, for example, a fiber optics gyroscope. An alternative possibility for determining the yaw angular velocity actual value (.mu..sub.ist) is provided by deriving the yaw angular velocity (.mu..sub.ist), with the use of at least one acceleration sensor which measures the radial acceleration of the vehicle. In addition, a yaw angular velocity required value (.mu..sub.soll) is derived from the measured velocity of the vehicle in the longitudinal direction and the measured steering angle. A critical handling situation is then deduced when the yaw angular velocity actual value (.mu..sub.ist) deviates from the yaw angular velocity required value (.mu..sub.soll), i.e. when the actual behavior of the vehicle deviates from the required behavior of the vehicle. This detected deviation of the actual behavior from the required behavior of the vehicle is then used to minimize the deviation by braking or accelerating individual wheels of the vehicle in such a way that the deviation is minimized.
DE 39 19 347 A1 describes a comparison between a desired yaw rate and the actual value of the yaw rate of a vehicle in order to influence the steering behavior of the vehicle by an appropriate action on the brakes. In this arrangement, the braking force on the wheels at the inside of the curve or on the wheels at the outside of the curve is influenced.
DE 38 17 546 A1 describes varying (i.e. increasing or reducing) the brake pressure on the wheels of a vehicle in pulses for a short period in order to deduce from the resulting transverse accelerations how far the corresponding wheel is removed from the stability limit. Slip thresholds are then correspondingly varied as a function of the distance from the stability limit. The distance from the stability limit is determined from the transverse accelerations since these transverse accelerations characterize the slope of the friction/slip curve. Conclusions on the distance from the stability limit can then be drawn from the order of magnitude of this slope because it becomes increasingly flatter as the distance from the stability limit becomes less.
A so-called linear single-track model of a vehicle is also known (DE-Buch: Zomotor, Adam; Fahwerktechnik: Fahrverhalten; Herausgeber: Jornsen Reimpell; Wurzburg: Vogel, 1987; First Edition; ISBN 3-8023-0774-7, in particular pages 99-127) by means of which, for example, a yaw angular velocity (.mu..sub.ist) of the vehicle occurring under certain conditions can be determined from measured values of the vehicle velocity in the vehicle longitudinal direction and the steering wheel angle or the steering angles of the wheels corresponding thereto. On the basis of this model, this yaw angular velocity (.mu..sub.ist) is then used as the yaw angular velocity required value (.mu..sub.soll).
An object of the present invention is to provide an apparatus and method for preventing vehicle handling instabilities in such a way that handling instabilities are, as far as possible, prevented before they occur.
This object has been achieved in accordance with the present invention by forming the time derivative of the difference in a computer unit with an output signal being generated in the computer unit as a function of this derivative with respect to time. The output signal contains information on whether the vehicle exhibits understeer or oversteer. The slip threshold value (.sigma..sub.soll) is varied in the direction of the value 0 in a computer device on the basis of the analysis of the output signal, i.e. is reduced in the case of drive slip and is increased when an engine braking torque occurs, if it is deduced from the analysis of the output signal that a changed lateral guidance force is required on the wheels of the drive axle of the vehicle. In vehicles with driven rear wheels a requirement for a changed lateral guidance force is deduced, for both oversteer and understeer in the case of drive slip and for oversteer when an engine braking torque occurs. In vehicles with driven front wheels a requirement for a changed lateral guidance force is deduced, for both oversteer and understeer in the case of drive slip and for understeer when an engine braking torque occurs.
One advantage of the present invention resides in the fact that, due to the early detection of the handling situation or of the yaw behavior of the vehicle, unstable handling conditions can be recognized even at an early stage. It is therefore possible to prevent the possible occurrence of unstable handling conditions even at an early stage by adapting the slip threshold values.
The vehicle longitudinal velocity and the steering wheel angle or the steering angles of the wheels are recorded by way of appropriate sensors. These sensor signals can then be supplied to a computer unit in which, from these parameters, a vehicle yaw angular velocity (.mu..sub.soll) desired by the driver can be determined as the yaw angular velocity required value (.mu..sub.soll) in accordance, for example, with the aforementioned linear single-track model. The handling situation or the yaw behavior is then detected in the computer unit by comparing the yaw angular velocity actual value .mu..sub.ist with the required value .mu..sub.soll which has been determined.
It is not only the magnitude of the difference between the yaw angular velocity actual value (.mu..sub.ist) and the required value (.mu..sub.soll) which is considered but also the sign of this difference and the time derivative of this difference. Particularly early recognition of the possible occurrence of critical handling situations is possible by taking account of, in particular, the derivative with respect to time so that the occurrence of critical handling situations can then be prevented at this point by appropriate variations of the slip threshold values.
The slip threshold value in the drive slip control system is varied as a function of the handling situation or vehicle yaw behavior detected. If a requirement for a larger lateral guidance force at the driving wheels is then deduced from the yaw behavior of the vehicle, the slip threshold values are correspondingly reduced.
The slip is calculated by subtracting the wheel rotational speed corresponding to the instantaneous vehicle speed from the measured wheel rotational speed. In this case, the slip is given in the physical units of the rotational speed. The difference determined in this way can, however, also be referred to the measured wheel rotational speed or to the wheel rotational speed corresponding to the instantaneous vehicle speed. In the latter case, the slip appears as a relative quantity which can, for example, be given as a percentage (%).
Instead of determining the yaw angular velocity required value (.mu..sub.soll) by way of the aforementioned linear single-track model, it is also possible to determine this required value from a previously determined characteristic diagram, i.e. a diagram determined on one occasion.